EFFECT OF HEAT TRANSFER SURFACE STRUCTURE ON WALL TEMPERATURE AND VOID FRACTION CHARACTERISTICS IN BOILING TRANSITION

摘要

This study deals with the departure from nucleate boiling (DNB) of subcooled flow boiling in a horizontal rectangular narrow channel. The effect of the structure of the heating surface, smooth and rough surface produced by thermal spray coating, on heat transfer and flow characteristics in boiling transition from nucleate to film boiling was examined. FC72 was used as the working fluid. The one side of the wall at the center of the narrow channel was replaced with the copper heating surface. The channel height was 4 mm, the channel width was 20 mm, and the heating length was 50 mm. The top wall was replaced with a transparent acrylic resin plate to observe the boiling flow behavior. Average void fraction was measured at the center of the channel by a capacitance probe in the boiling transition from nucleate to film boiling. As the results, the coating produced higher critical heat flux than the smooth surface. A large fluctuation of pressure in synchronization with void fraction fluctuation was observed just before the boiling transition. The fluctuation was caused by periodic change in boiling behavior of nucleate and film boiling. During the pressure fluctuation, the wall temperature gradually increased, and then a stable vapor film was formed. While the wall temperature at the start of the fluctuation for the coated surface was lower than the smooth surface, the effect of the surface structure on the wall temperature at the stable vapor film formation was little.